Gate driver and power converter

ABSTRACT

A gate driver includes: a timing determination unit configured to measure an on-time of a switching element and configured to determine, based on the on-time, a certain timing during a turn-off period of the switching element as an intermediate timing; and a driving condition changing unit configured to change a gate driving condition of the switching element at the intermediate timing determined by the timing determination unit.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of U.S. patentapplication Ser. No. 16/800,083 filed on Feb. 25, 2020, which is basedupon and claims priority to Japanese Patent Application No. 2019-116691,filed on Jun. 24, 2019. The entire contents of the applications areincorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a gate driver and a power converter.

2. Description of the Related Art

Conventionally, in order to reduce surge voltage and switching losses,an active gate driving technique is known, in which switching speed ischanged at an appropriate timing in accordance with drain current orcollector current (which may be referred to as main current hereinafter)flowing through a switching element. For example, Patent Document 1discloses a gate drive circuit that stores a surge period from a timingof a turn-off command to an occurrence timing of voltage surge. At thetime of current turn-off, the gate drive circuit determines a timing ofchanging effective gate resistance of the switching element, based onthe surge period stored at the time of previous turn-off.

On the other hand, in a power converter such as a chopper circuit, thereis known a technique for using, depending on the purpose, a currentcontinuous mode in which current flowing through a reactor is continuousand a current discontinuous mode in which current flowing through areactor is intermittent (see, for example, Patent Document 2).

RELATED-ART DOCUMENTS Patent Document

-   [Patent Document 1] Japanese Patent No. 4935266-   [Patent Document 2] Japanese Patent No. 6398537

In the conventional active gate driving technique, it is assumed thatthe present current value of the main current at the start time point ofturn-off is the same as the previous current value of the main currentat the start time point of turn-off. However, in a driving mode (e.g., acurrent discontinuous mode) in which the current value of the maincurrent at the start time point of turn-off may differ between thepresent time and the previous time, a timing that is appropriate forvarying the switching speed to achieve suppression of the switch-offsurge voltage and reduction of the switching losses varies on acase-by-case basis. Accordingly, active gate driving cannot be performedat an appropriate time, and it may be difficult to both suppress theswitch-off surge voltage and reduce the switching losses.

Accordingly, the present disclosure provides a gate driver and a powerconverter that can achieve both suppression of switch-off surge voltageand reduction of switching losses, by changing the switching speed at anappropriate timing.

SUMMARY OF THE INVENTION

The present disclosure provides a gate driver including: a timingdetermination unit configured to measure an on-time of a switchingelement and configured to determine, based on the on-time, a certaintiming during a turn-off period of the switching element as anintermediate timing; and a driving condition changing unit configured tochange a gate driving condition of the switching element at theintermediate timing determined by the timing determination unit.

According to the present disclosure, because it is possible to changethe switching speed at an appropriate timing, it is possible to providea gate driver and a power converter that can achieve both suppression ofswitch-off surge voltage and reduction of switching losses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration example of a gatedriver;

FIG. 2 is a circuit diagram illustrating a configuration of a choppercircuit that is an example of a power converter;

FIG. 3 is a diagram illustrating an example of operation waveforms of acurrent continuous mode and a current discontinuous mode;

FIG. 4 is a timing chart illustrating a first operation example of thegate driver; and

FIG. 5 is a timing chart illustrating a second operation example of thegate driver.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following, an embodiment according to the present disclosure willbe described with reference to the drawings.

FIG. 1 is a block diagram illustrating a configuration example of a gatedriver 1. The gate driver 1 illustrated in FIG. 1 is a circuit thatprovides positive or negative voltage to a gate of a switching element11 to turn on/off the gate of the switching element 11. The gate driver1 drives the gate of the switching element 11 using an active gatedriving technique that adjusts switching speed of the switching element11 during turn-off of the switching element 11.

The switching element 11 is a voltage driven semiconductor elementhaving a control electrode (gate), a first main electrode (collector ordrain), and a second main electrode (emitter or source). Examples of theswitching element 11 include a metal oxide semiconductor field effecttransistor (MOSFET), an insulated gate bipolar transistor (IGBT), andthe like. FIG. 1 illustrates a case in which each of the switchingelement 11 is an N-channel MOSFET.

The switching element 11 may be made of a semiconductor such as Si(silicon), or may be a wide-bandgap device made of a wide-bandgapsemiconductor such as SiC (silicon carbide), GaN (gallium nitride),Ga₂O₃ (gallium oxide), or diamond. Applying a wide-bandgap device to theswitching element 11 enhances an effect of loss reduction of theswitching element 11.

The gate driver 1 includes, for example, a gate drive unit 200, a timingdetermination unit 241, and a driving condition changing unit 231.

The gate drive unit 200 is a circuit unit that drives a gate of theswitching element 11 in accordance with an input signal from outside ofthe gate driver 1. The input signal is a signal that commands switchingof the switching element 11, and, for example, is a pulse-widthmodulated signal (PWM signal). In a case in which a PWM signal is usedas the input signal, the input signal at an active level (e.g., a highlevel) represents an on-command to turn on the switching element 11, andthe input signal at an inactive level (e.g., a low level) represents anoff-command to turn off the switching element 11.

The timing determination unit 241 measures an on-time width T_(ON) fromwhen the input signal is switched to the on-command to when the inputsignal is switched to the off-command, and determines, based on themeasured on-time width T_(ON), an intermediate timing that is beforeswitch-off surge voltage of the switching element 11 reaches the peak.The intermediate timing tm is a condition changing timing at which thegate driving condition of the switching element 11 is changed duringturn-off so that the switching speed of the switching element 11 variesduring turn-off.

The timing determination unit 241 includes, for example, a time widthmeasurement unit 211 and a timing output unit 221.

The time width measurement unit 211 measures the on-time width T_(ON)from when the input signal is switched to the on-command to when theinput signal is switched to the off-command. The time width measurementunit 211 may be configured to measure the on-time width T_(ON) by acounter or a filter or may be configured to convert the pulse width ofthe input signal to a voltage value.

The timing output unit 221 outputs a timing signal S to cause thedriving condition changing unit 231 to change the gate driving conditionof the switching element 11 at the intermediate timing tm in accordancewith the on-time width T_(ON) measured by the time width measurementunit 211.

In accordance with the timing signal S output from the timing outputunit 221, the driving condition changing unit 231 changes the gatedriving condition of the switching element 11 at the intermediate timingtm determined by the timing determination unit 241. Although drivingconditions a1 and a2 having different conditions are exemplified as thegate driving conditions in FIG. 1, three or more different drivingconditions may be set.

The driving condition changing unit 231 selects one of the drivingconditions a1 and a2 in accordance with the timing signal S. Forexample, the driving condition changing unit 231 selects the drivingcondition a1 during a period in which the timing signal S is output fromthe timing determination unit 241, and selects the driving condition a2during a period in which the timing signal S is not output from thetiming determination unit 241.

The driving condition changing unit 231 includes, for example, two gateresistors each having a different resistance value, and a switch circuitthat switches whether to connect each gate resistor to the gate of theswitching element 11. The resistance value of the gate resistor that isconnected to the gate of the switching element 11 when the drivingcondition a1 is selected is smaller than the resistance value of thegate resistor that is connected to the gate of the switching element 11when the driving condition a2 is selected.

Accordingly, during turn-off of the switching element 11 by the gatedrive unit 200, by selecting the driving condition a1 in which theresistance value of the gate resistor decreases, the switching speed(turn-off speed) of the switching element 11 increases. Therefore,switching losses at the time of turn-off can be reduced. Meanwhile,during turn-off of the switching element 11 by the gate drive unit 200,by selecting the driving condition a2 in which the resistance value ofthe gate resistor increases, switching speed (turn-off speed) of theswitching element 11 decreases. Therefore, a change rate (dI/dt) ofdrain current flowing through the switching element 11 with respect totime decreases, and switch-off surge voltage can be suppressed.

The driving condition changing unit 231 may also be configured toinclude two current sources each having a different current value and aswitch circuit that switches whether to connect each gate current sourceto the gate of the switching element 11. The current value of the gatecurrent source connected to the gate of the switching element 11 whenthe driving condition a1 is selected is greater than the current valueof the gate current source connected to the gate of the switchingelement 11 when the driving condition a2 is selected.

Accordingly, during turn-off of the switching element 11 by the gatedrive unit 200, by selecting the driving condition a1 in which thecurrent value of gate current increases, switching speed (turn-offspeed) of the switching element 11 increases. Therefore, switchinglosses at the time of turn-off can be reduced. Meanwhile, duringturn-off of the switching element 11 by the gate drive unit 200, byselecting the driving condition a2 in which the current value of gatecurrent decreases, switching speed (turn-off speed) of the switchingelement 11 decreases. Therefore, a change rate (dI/dt) of drain currentflowing through the switching element 11 with respect to time decreases,and switch-off surge voltage can be suppressed.

Here, the intermediate timing tm that is before the switch-off surgevoltage of the switching element 11 reaches the peak will be describedwith reference to FIG. 2 and FIG. 3.

FIG. 2 is a diagram illustrating a configuration of a chopper circuitthat is an example of a power converter. FIG. 3 illustrates operationwaveforms of a current continuous mode and a current discontinuous mode.In the current continuous mode and the current discontinuous mode, amain current (drain current or collector current) flowing through thesemiconductor element S1 is proportional to an input signal (e.g., dutyratio command) to a gate driver that drives a semiconductor element S1in the power converter. In FIG. 2, E1 indicates an input voltage, E2indicates an output voltage, L indicates a reactor, and I_(L) indicatesa reactor current. In a case of this booster circuit, because the draincurrent is equal to the reactor current, the drain current changesaccording to the following equations.When S1 is on: Δi _(L) =E ₁ /L×T _(ON)When S1 is off: Δi _(L)=(E ₁ −E ₂)/L×T _(OFF)

Δi_(L) indicates an amount of change in drain current, T_(ON) indicatesan on-time of the semiconductor element S1, and T_(OFF) indicates anoff-time of the semiconductor element S1. Note that i_(L) is not lessthan or equal to 0 due to a diode D.

In the current discontinuous mode, as illustrated in FIG. 3, because thereactor current i_(L) becomes 0 during one cycle of a carrier, thecurrent value of the reactor current i_(L) (drain current) immediatelybefore the turn-off (in other words, at the start time point ofturn-off) depends on the on-time width T_(ON). Therefore, thepeak-reaching time from the turn-off start timing of the semiconductorelement S1 to the timing at which the switch-off surge voltage becomesthe peak value also roughly depends on the on-time width T_(ON).

Therefore, by using the measured value of the on-time width T_(ON), thetiming determination unit 241 can determine the intermediate timing tmthat is before the switch-off surge voltage of the semiconductor elementS1 reaches the peak. The driving condition changing unit 231 changes thegate driving condition of the switching element 11 at the intermediatetiming tm determined by the timing determination unit 241. That is, thetiming determination unit 241 can determine the intermediate timing tmthat is appropriate for varying the switching speed for suppressing thesurge voltage and reducing the switching losses. Because the gatedriving condition can be changed at an appropriate intermediate timingtm, it is possible to both suppress the surge voltage and reduce theswitching losses. By achieving both suppression of the surge voltage andreduction of the switching losses, for example, it is possible to reducethe size and cost of a magnetic element such as a transformer or areactor, and it is possible to reduce the size of a cooling body of theswitching element.

In particular, in the current discontinuous mode, the present currentvalue of the main current at the turn-off start time point may differfrom the previous current value of the main current at the turn-offstart time point. Therefore, it is advantageous to determine anappropriate timing tm as described above.

FIG. 4 is a timing chart illustrating a first operation example of thegate driver 1. According to an input signal that causes the switchingelement 11 to be switched in the current discontinuous mode, the gatedrive unit 200 supplies a control signal (gate driving signal) to acontrol terminal (gate) to the switching element 11 via the drivingcondition changing unit 231. In this example, the input signal at thehigh level represents an on-command of the switching element 11, and theinput signal at the low level represents an off-command of the switchingelement 11.

In a case in which the input signal is changed from the off-command tothe on-command, the switching element 11 starts turning on in accordancewith the control signal input to the control terminal (at time t1).Drain-source voltage VDS of the switching element 11 starts decreasing,and drain current Id starts increasing. The time width measurement unit211 starts measuring the on-time width T_(ON) of the input signal at thesame time as the input signal is changed to the on-command. For example,while the input signal is the on-command, the time width measurementunit 211 down-counts with a predetermined count start value as a startpoint and converts the on-time width T_(ON) of the input signal to anumerical value or a voltage value.

At this time, the time width measurement unit 211 counts at a countingspeed such that the on-time width T_(ON) can be converted to anappropriate intermediate timing tm at which both suppression of surgevoltage and reduction of switching losses can be achieved. The countingspeed of the time width measurement unit 211 is a value that is set inadvance so as to satisfy a relationship X between the on-time widthT_(ON) and an appropriate intermediate timing tm at which bothsuppression of surge voltage and reduction of switching losses can beachieved.

Note that the time width measurement unit 211 may count with a countstart value as a start point such that the on-time width T_(ON) can beconverted to an appropriate intermediate timing tm at which bothsuppression of surge voltage and reduction of switching losses can beachieved. At this time, the count start value of the time widthmeasurement unit 211 is a value that is set in advance so as to satisfya relationship X between the on-time width T_(ON) and an appropriateintermediate timing tm at which both suppression of surge voltage andreduction of switching losses can be achieved.

Thereafter, when the input signal changes from an on-command to anoff-command, the switching element 11 starts turning off (at time t2) inaccordance with the control signal input to the control terminal. At thesame time as the input signal is changed to the off-command, the timewidth measurement unit 211 stops counting and outputs a signalrepresenting the magnitude of the on-time width T_(ON) of the inputsignal to the timing output unit 221.

The timing output unit 221 outputs the timing signal S according to theoutput of the time width measurement unit 211. In FIG. 4, the countvalue of the time width measurement unit 211 at time t2 when the inputsignal becomes the off command is the count start value of the timingoutput unit 221. The timing output unit 221 down-counts at apredetermined constant counting speed and outputs the timing signal Sduring time period Δt0 to time t3 when the count value becomes zero.Here, t3 corresponds to an appropriate intermediate timing tm at whichit is possible to both suppress surge voltage and reduce switchinglosses.

That is, at t2 when the turn-off operation is started, due to startingoutputting the timing signal S, the driving condition is switched fromthe driving condition a2 to the driving condition a1, and thedrain-source voltage VDS starts increasing. Because the drivingcondition is switched from the driving condition a2 to the drivingcondition a1, the switching speed increases during the first half of theturn-off period and the switching losses decrease. For example, thedriving condition changing unit 231 may increase a resistance value ofthe gate resistor connected to the gate of the switching element 11 attime t2, or may decrease a current value of the gate current flowinginto the gate of the switching element 11 at time t2.

Then, upon the miller plateau region of the switching element 11 ending(at time t3), at the time of the drain current starting rapidlydecreasing, the surge voltage corresponding to the change rate of thedrain current with respect to time occurs. However, by the count valueof the timing output unit 221 becoming zero, the output of the timingsignal S stops. Accordingly, because the gate driving condition isswitched from the driving condition a1 to the driving condition a2 attime t3, the switching speed during the second half of the turn-offperiod is reduced, and the switch-off surge voltage can be suppressed.For example, the driving condition changing unit 231 may increase aresistance value of the gate resistor connected to the gate of theswitching element 11 at time t3, or may decrease a current value of thegate current flowing into the gate of the switching element 11 at timet3.

As described above, the time width measurement unit 211 outputs thecount value corresponding to the on-time width T_(ON) by counting fromwhen the input signal is switched to an on-command to when the inputsignal is switched to an off-command. The timing output unit 221changes, in accordance with the count value, the length of the periodΔt0 during which the timing signal S is output. Thus, even if the lengthof the on-time width T_(ON) varies with each turn-off timing, anappropriate timing tm can be set so that both suppression of surgevoltage and reduction of switching losses can be achieved. For example,as illustrated in FIG. 4, at the next ((n+1)-th) switching timing, evenwhen the length of the on-time width T_(ON) becomes longer than that ofthe present (n-th) switching timing, the length of the period Δt1 foroutputting the timing signal S can be reduced.

Thereby, it is possible to set the appropriate timing tm.

Also, the intermediate timing tm is the timing t3 within the period ofthe off-command adjacent to the measured on-time width T_(ON).Therefore, even in the current discontinuous mode, it is possible to setthe intermediate timing tm that is appropriate for the magnitude of thedrain current that flows at the previous on-time width T_(ON).

FIG. 5 is a timing chart illustrating a second operation example of thegate driver 1. Because operations other than processes between the timewidth measurement unit 211 and the timing output unit 221 are the sameas the first operation example, the description thereof is omitted orsimplified by incorporating the above-described description. In thefirst operation example, the time width measurement unit 211 countsbased on the relationship X, while in the second operation example, thetiming output unit 221 counts based on the relationship X. They differin this respect.

In a case in which the input signal is changed from the off-command tothe on-command, the switching element 11 starts turning on in accordancewith the control signal input to the control terminal (at time t1). Thetime width measurement unit 211 starts measuring the on-time widthT_(ON) of the input signal at the same time as the input signal ischanged to the on-command. For example, while the input signal is theon-command, the time width measurement unit 211 down-counts with apredetermined count start value (which is 0, in this case) as a startpoint and converts the on-time width T_(ON) of the input signal to anumerical value or a voltage value.

At this time, the time width measurement unit 211 does not have to countat a counting speed that enables to convert the on-time width T_(ON) toan appropriate intermediate timing tm at which both suppression of surgevoltage and reduction of switching losses can be achieved. For example,the counting speed of the time width measurement unit 211 may be a valuethat is set in advance in accordance with an actual passage of time.

Thereafter, when the input signal changes from an on-command to anoff-command, the switching element 11 starts turning off (at time t2) inaccordance with the control signal input to the control terminal. At thesame time as the input signal is changed to the off-command, the timewidth measurement unit 211 stops counting and outputs a signalrepresenting the magnitude of the on-time width T_(ON) of the inputsignal to the timing output unit 221.

The timing output unit 221 outputs the timing signal S according to theoutput of the time width measurement unit 211. For example, withreference to a relationship between the on-time width T_(ON) and theintermediate timing tm, at which both suppression of surge voltage andreduction of switching losses can be achieved, the timing output unit221 outputs, in accordance with the on-time width T_(ON) measured by thetime width measurement unit 211, the timing signal S so as to satisfythe relationship.

In FIG. 5, as a first example, the count value of the time widthmeasurement unit 211 at time t2 when the input signal becomes theoff-command is the count start value of the timing output unit 221. Thetiming output unit 221 down-counts at a counting speed corresponding tothe count value of the time width measurement unit 211 at time t2 whenthe input signal becomes off, and outputs the timing signal S duringtime period Δt0 to time t3 when the count value becomes zero. Here, t3corresponds to an appropriate intermediate timing tm at which it ispossible to both suppress surge voltage and reduce switching losses.That is, the timing output unit 221 outputs the timing signal S duringthe period from the start to the end of counting at the counting speedcorresponding to the on-time width T_(ON) measured by the time widthmeasurement unit 211.

At this time, the counting speed of the timing output unit 221 is avalue that is set sequentially in accordance with the on-time widthT_(ON) measured for each turn-on operation so as to satisfy therelationship X between the on-time width T_(ON) and the appropriateintermediate timing tm at which both suppression of surge voltage andreduction of switching losses can be achieved. For example, the timingoutput unit 221 determines, based on the relationship X, the countingspeed corresponding to the on-time width T_(ON) measured by the timewidth measurement unit 211 and outputs the timing signal S during theperiod from the start to the end of counting at the determined countingspeed.

Alternatively, in FIG. 5, as a second example, the timing output unit221 may down-count with a count start value as a start pointcorresponding to the count value of the time width measurement unit 211at time t2 when the input signal becomes off. In this case, the countingspeed of the timing output unit 221 may be a predetermined constantvalue and is a value that is set in advance in accordance with an actualpassage of time, for example. That is, the timing output unit 221outputs the timing signal S during the period from the start to the endof counting at the counting speed corresponding to the on-time widthT_(ON) measured by the time width measurement unit 211.

At this time, the count start value of the timing output unit 221 is avalue that is set sequentially in accordance with the on-time widthT_(ON) measured for each turn-on operation so as to satisfy therelationship X between the on-time width T_(ON) and the appropriateintermediate timing tm at which both suppression of surge voltage andreduction of switching losses can be achieved. For example, the timingoutput unit 221 determines, based on the relationship X, the count startvalue corresponding to the on-time width T_(ON) measured by the timewidth measurement unit 211 and outputs the timing signal S during theperiod from the start to the end of counting at the determined countstar value.

That is, at t2 when the turn-off operation is started, due to startingoutputting the timing signal S, the driving condition is switched fromthe driving condition a2 to the driving condition a1, and thedrain-source voltage VDS starts increasing. Because the drivingcondition is switched from the driving condition a2 to the drivingcondition a1, the switching speed increases during the first half of theturn-off period and the switching losses decrease.

Then, upon the miller plateau region of the switching element 11 ending(at time t3), at the time of the drain current starting rapidlydecreasing, the surge voltage corresponding to the change rate of thedrain current with respect to time occurs. However, by the count valueof the timing output unit 221 becoming zero, the output of the timingsignal S stops. Accordingly, because the gate driving condition isswitched from the driving condition a1 to the driving condition a2 attime t3, the switching speed during the second half of the turn-offperiod is reduced, and the switch-off surge voltage can be suppressed.

In this manner, in a case in which the counting speed of the time widthmeasurement unit 211 is not a value that is previously set to satisfythe above described relationship X, the count start value or thecounting speed of the timing output unit 221 may be changed inaccordance with the on-time width T_(ON) measured by the time widthmeasurement unit 211. At this time, the timing output unit 221 maydetermine the count start value or the counting speed on a case-by-casebasis in accordance with an appropriate timing tm for changing thedriving condition with respect to the on-time width T_(ON) measured bythe time width measurement unit 211. Note that the method of referringto the relationship X described above may be another method.

Although the gate driver and the power converter have been describedwith reference to the embodiment, the present invention is not limitedto the embodiment described above. Various modifications andenhancements, such as combinations and substitutions with some or all ofthe other embodiments, are possible within the scope of the presentinvention.

For example, a power converter including at least one gate driver is notlimited to a DC-DC converter that converts direct current to anotherdirect current. Specific examples of the power converter include aninverter for converting direct current to alternating current, a step-upconverter for increasing an input voltage and outputting the increasedvoltage, a step-down converter for decreasing an input voltage magnitudeand outputting the decreased voltage, and a buck-boost converter forincreasing or decreasing an input voltage and outputting the increasedor decreased voltage.

For example, functional elements as described above may be realized by amemory that stores at least one program and by a processor coupled tothe memory.

What is claimed is:
 1. A gate driver comprising: a timing determinationunit configured to measure an on-time of a switching element andconfigured to determine, based on the on-time, a certain timing during aturn-off period of the switching element as an intermediate timing; anda driving condition changing unit configured to change a gate drivingcondition of the switching element at the intermediate timing determinedby the timing determination unit.
 2. The gate driver according to claim1, wherein the driving condition changing unit is configured to decreasea current value of gate current flowing into a gate of the switchingelement at the intermediate timing.
 3. The gate driver according toclaim 1, wherein the switching element is a wide-bandgap device.
 4. Apower converter comprising: the gate driver according to claim 1 and theswitching element.